Ch-19 Forming and shaping plastics

1. Ch-19 Forming and shaping plastics Thermoplastic (TP) – Polymers that can be shaped when heated and regain original hardness & strength upon cooling – Have a linear or branched structure (weak secondary bonds) – Process is reversible – Acrylics, cellulosics, nylons, polyethylenes, polyvinyl chloride • Thermoset (TS) – Polymers that become permanently set when heated – Have a cross-linked structure (strong secondary bonds) – Process is irreversible – Epoxy, polyester, urethane, phenolics, silicones • Elastomer (Rubber) – Elastic; low elastic modulus – Tires, footwear, gaskets, flooring, weather stripping, hoses

2. Forming and Shaping Processes for Plastics, Elastomers, and Composite Materials Figure 19.1 Outline of forming and shaping processes for plastics, elastomers, and composite materials. (TP = Thermoplastics; TS = Thermoset; E = Elastomer.)

3. Forming and shaping plastics - Extrusion • raw materials in form of thermoplastic (TP) pellets, granules, or powder are placed into a hopper and fed into the extruder barrel (F19.2). • barrel is equipped with a screw that blends pellets & conveys them down the barrel. • internal friction from the mechanical action of the screw, along with heaters around the extruder’s barrel, heats the pellets and liquefies them. • Screws have 3 distinct sections: • Feed section that conveys material from hopper area into central region of barrel. • Melt section: heat generated by shearing of plastic and by heaters causes melting to begin. • Pumping section: additional shearing and melting occurs, with pressure buildup at die.

4. Forming and shaping plastics - Extrusion

5. Forming and shaping plastics - Extrusion • Molten plastic or elastomer is forced through a die. • The extruded product is then cooled, either by exposing it to air or by passing it through a water-filled channel. • The extruded product is then coiled or cut into desired lengths. • Typical products: solid rods, channels, tubing, pipe, window frames, sheet, Pellets for other plastic processing methods • Plastic-coated electrical wire, and cable are also extruded. The wire is fed into the die opening at a controlled rate with the extruded plastic. • Extruders can also be used as simple melters for injection molding and blow molding. • Process parameters: • extruder-screw speed • barrel-wall temp • die design • cooling and drawing speeds

6. Forming and shaping plastics - Extrusion

7. Plastic Tubes and Pipes Extrusion Figure 19.4 Extrusion of tubes. (a) Extrusion using a spider die (see also Fig. 15.8) and pressurized air. (b) Coextrusion for producing a bottle.

8. Extrusion - Thin Polymer Films • Extruders are rated by diameter D of barrel & length to diameter (L/D) ratio of barrel. • Typical commercial units: D=25-200mm, L/D = 5-30. Extrusion equipment cost: $300,000. • Thin polymer films and common plastic bags are made from a tube produced by an extruder (F18.4). • in this process, a thin walled tube is extruded vertically upward. • then expanded into a balloon shape by blowing air through the center of the extrusion die until desired film thickness is reached.

9. Extrusion - Thin Polymer Films FIGURE l9.5 (a) Schematic illustration of the production of thin film and plastic bags from tube-first produced by an extruder and then blown by air. (b) A blown-film operation. This process is well developed, producing inexpensive and very large quantities of plastic film and shopping bags. Source: (b) Courtesy of Wind Moeller 86 Hoelscher Corp.

10. Injection molding (IM) • Pellets or granules are fed into the heated cylinder, and the melt is forced into a split die chamber, either by hydraulic plunger or by a rotating screw system of an extruder. • After part is sufficiently cooled, molds are opened and part is ejected. • IM pressures: 70-200MPa • Typical products: cups, containers, housings, tool handles, knobs, electrical components, toys, plumbing fittings.

11. Injection molding (IM)

12. Injection molding (IM) • Complex shapes and Good dimensional accuracy. • 3 basic types of molds: • cold runner 2-plate mold: solidified plastic in channels that connect the mold cavity to end of barrel must be trimmed. • cold-runner 3-plate: runner system is separated from part when mold opens. • Hot-runner mold: more expensive, no gates, runners, or sprues attached to molded part. Molten plastic is kept hot in a heated runner plate. Shorter cycle times. • Insert molding: metallic components such as screws, pins, and strips.

13. Products Made by Injection Molding (a) (b) Figure 19.9 Typical products made by injection molding, including examples of insert molding. Source: (a) Courtesy of Plainfield Molding, Inc. (b) Courtesy of Rayco Mold and Mfg. LLC.

14. Injection molding (IM) • Process capabilities: • Good dimensional control. • Cycle times: 5-60sec, several min for thermoset (TS) materials. • Mold materials: tool steels, beryllium, Cu, or Al. • Mold costs: up to $100,000 • Mold life: up to 2M cycles for steel molds, 10,000 cycles for Al molds. • Machines: horizontal or vertical (small close-tolerance parts and for insert molding) • Injection molding machines are rated according to the capacity of the mold and the clamping force. Force: 0.9-2.2MN. • Cost of a 100 ton machine: $60,000-90,000. • Cost of a 300 ton machine: $85,000-140,000. • Cost of dies: $20,000-200,000.

15. Reaction injection molding (RIM) • A mixture of resin with two or more reactive fluids is forced into the mold cavity at high speed (F18.8). • Chemical reactions take place rapidly in the mold, and the mold solidifies into a thermoset part. • Typical parts: automotive bumpers and fenders, thermal insulation for ref and freezers.

16. Structural foam molding (SFM) • SFM process is used to make plastic products with a solid outer skin and a cellular inner structure. • Typical products: furniture components, TV cabinets, business machine housings. • Injection foam molding: • thermoplastics are mixed with a blowing agent (inert gas such as N2), which expands the material. • The core of part is cellular, and the skin is rigid. • Thickness of skin: up to 2 mm. • Part densities as low as 40% of the density of solid plastic.

17. blow molding • Extrusion blow molding: • a tube is first extruded or injection moldod, • Then clamped into a mold cavity much larger than tube diameter. • Finally blown outward to fill the mold cavity (F18.9a). • Blowing is usually done with an air blast at a pressure of 350-700KPa. • The molds close around the tubing, close off both ends (thereby breaking the tube into sections), and then move away as air is injected into the tabular piece. • The part is then cooled and ejected. • Typical parts: beverage bottles and hollow containers.

18. rotational molding • Thin walled metal mold is made of 2 pieces and is designed to be rotated about 2 per axes (F18.10). • A pre-measured quantity of powdered plastic material is placed inside the worm mold. • The mold is then heated, while rotated about the 2 axes. • This action tumbles the powder against the mold, where heating fuses the powder without melting it. • Typical parts: toys, carrying cases, footballs.

19. thermoforming • a sheet is heated in oven to the sag point. • Sheet is then removed from the oven, placed over a mold, and forced against the mold through the application of a vacuum. • The mold is usually at RT, hence the shape of the plastic becomes set upon contact with the mold. • Typical parts: ad signs refrigeration liners, packaging, and appliance housings. • Molds: AL. The holes in molds are usually less than 0.5mm, in order not to leave any marks on the formed parts.

20. compression molding (CM) • A pre-shaped charge of material, a pre-measured volume of powder, or a viscous mixture of liquid resin and filler material is placed directly into a heated mold cavity. • Forming is done under pressure from upper half of die (F18.12). • CM is used mainly with thermosetting plastics, with original material being in a partially polymerized state. X-linking is completed in the heated die. • Curing times: 0.5-5min. • Typical parts: dishes, handles, container caps, fittings. • 3 types of compression molds: • Flash type: for shallow or flat parts. • positive: for high density parts • Semi positive: for quality prod.

21. Transfer Molding (TM) • Uncured TS material is placed in a heated transfer pot or chamber (F18.13). • After material is heated, it’s injected into heated closed molds. Curing takes place by x-linking. • Typical parts: electrical and electronic comp, rubber and silicon parts. • TM Suitable for intricate shapes with varying wall thicknesses.

22. casting • Typical parts: gears, bearings, wheels, thick sheets. • convention casting of TP: a mixture of monomer, catalyst, and various additives is heated and poured into the mold. Part forms after polymerization takes place at ambient pressure. • centrifugal casting • Potting and encapsulation: potting (F18.14b) is done in a housing or case, which is an integral part of product. In encapsulation (F18.14c), comp is coated with a layer of the solidified plastic • Foam molding and casting:. Polystyrene beads are placed in a mold with a blowing agent and exposed to heat, usually by steam. As a result, the beads expand to as much as 50 times their original size and take the shape of the mold. Typical products: Styrofoam cups, food containers, insulating blocks, and shaped packaging materials. • Polyurethane foam processing: it starts with mixing of two or more chemical components, the reaction forms cellular structure which solidifies in the mold.

23. cold forming and solid phase forming • Rolling, deep drawing, extrusion, closed die forging, coining, and rubber forming, can also be used to form many TP at RT. • Important considerations: • ductility at RT • Material’s deformation must be non-recoverable (SB). • Advantages of cold forming • Strength, toughness, and uniform elongation are increased. • plastics with high molecular weights can be used • Forming speeds are not affected by part thickness because there is no heating or cooling involved. • Cycle times shorter than molding processes. • Solid phase forming: carried out at a temp from 10 to 20oC below melting temp of plastic, while it’s still in a solid state.